This invention relates to the production of metal matrix composites, and more particularly to methods of producing cast aluminum alloy composite articles.
Among metal matrix composites (MMC) having important commercial utility are fibre-reinforced articles of aluminum and its alloys, particularly aluminum-silicon alloys. One of the most popular techniques used to manufacture metal matrix composites is melt infiltration. In this procedure a preform of preferably fibrous alumina reinforcing material is infiltrated under pressure by liquid metal. The composite is then allowed to solidify by cooling. The resulting microstructure of the metal matrix is generally not the same as that found in non-reinforced castings.
If the cooling rate of an A1-Si casting is such that the free growth dendrite arm spacing is greater than the average fibre spacing, the metal matrix dendrites will be in the order of this size as they grow avoiding the alumina fibres. This leads to the rejected solute accumulating at the fibres. For A1-Si alloys the solute build-up is comprised of large silicon particles. These large silicon particles have poor physical properties (brittle, different coefficient of thermal expansion) and degrade the ultimate performance of the composite.
In the case where the cooling rate is high enough to ensure the average dendrite size is less than the average fibre spacing, the metal matrix microstructure appears identical to that in the non-reinforced region. However, large casting cross sections of greater than about 20 mm make it impossible to ensure a high enough cooling rate to keep the dendrite size less than the fibre spacing.
It has been known for many years to obtain a fine eutectic structure in A1-Si alloys containing about 5 to 15% silicon, by the use of additives and, thus, to improve the mechanical properties of these alloys. For instance, it is well known to use alkali metals and alkaline-earth metals, e.g. sodium or strontium, as additives in A1-Si alloys. These chemical additions to a melt reduce the silicon size by affecting the normal growth kinetics of the solidification process. It would, therefore, be expected that in a similar manner additives such as sodium or strontium would suitably modify the metal matrix microstructure of a metal matrix composite. However, when the melt contains a fibrous preform reinforcement, sodium and strontium are remarkably ineffective in modifying the metal matrix microstructure of the metal matrix composite. The sodium appears to be totally ineffective, while strontium can be used only with difficulty.
As a consequence, metal matrix composites typically contain large silicon particles and/or large intermetallics which tend to filter out and thereby accumulate at the preform/alloy melt interface during infiltration. These large silicon particles and intermetallics degrade the properties significantly at the composite/alloy interface and to a lesser extent, in the entire composite. For many uses of the metal matrix composites, this loss of properties can be tolerated. However, if the metal matrix composites are to be used in high stress situations where thermal fatigue is a major consideration, the loss of properties cannot be tolerated.
It is the object of the present invention to develop a process for forming a composite cast article in which adequate refining or modification of the eutectic silicon will occur within the preform.